1. Field of the Invention
The invention concerns an arrangement for thermal actuation of a mirror in a microlithographic projection exposure apparatus.
2. State of the Art
Microlithography is used for the production of microstructured components such as for example integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination system and a projection objective. In that case the image of a mask (=reticle) illuminated via the illumination system is projected via the projection objective onto a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective in order to transfer the mask structure onto the light-sensitive coating on the substrate.
Mirrors are used as optical components for the imaging process in projection objectives designed for the EUV range, that is to say at wavelengths of for example about 13 nm or about 7 nm, due to the lack of availability of suitable translucent refractive materials. A problem which arises in practice is that the EUV mirrors experience a rise in temperature and linked thereto thermal expansion or deformation, as a consequence of absorption of the radiation emitted by the EUV light source, and that expansion or deformation in turn can result in worsening of the imaging properties of the optical system. To assess those effects and possibly to be able to compensate for them there is a need to determine the extent of that rise in mirror temperature as accurately as possible and possibly control it in the sense of thermal actuation. In that respect in practice the further problem can arise that, by virtue of using special illumination settings (such as for example dipole or quadrupole settings) in the lithography process and by virtue of the diffraction orders caused by the reticle, the heat input caused by the EUV radiation can vary over the optically effective cross-section of mirrors near the pupil, that is to say that involves non-homogenous heat input into the mirror. In addition field variations in the reticle and/or partial masking off of the full field can result in non-homogenous light intensities on mirrors near the field.
Approaches for mirror temperature measurement and/or actuation of a mirror or targeted deformation thereof are known for example from WO 2010/018753 A1, US 2004/0051984 A1, WO 2008/034636 A2, DE 10 2009 024 118 A1 and WO 2009/046955 A2.